PHPIDS

PHPIDS (PHP-Intrusion Detection System) is a simple to use, well
structured, fast and state-of-the-art security layer for your PHP based
web application. The IDS neither strips, sanitizes nor filters any
malicious input, it simply recognizes when an attacker tries to break
your site and reacts in exactly the way you want it to. Based on a set
of approved and heavily tested filter rules any attack is given a
numerical impact rating which makes it easy to decide what kind of
action should follow the hacking attempt. This could range from simple
logging to sending out an emergency mail to the development team,
displaying a warning message for the attacker or even ending the user's
session.

PHPIDS enables you to see who's attacking your site and how and all
without the tedious trawling of logfiles or searching hacker forums for
your domain. Last but not least it's licensed under the LGPL!

http://php-ids.org/